The characteristic constant for a gas is given as:

The characteristic constant for a gas is expressed in terms of energy per unit mass per unit temperature change, which can be represented with the units kilojoules per kilogram per Kelvin (kJ/kg K).

When dealing with the ideal gas law or thermodynamic calculations involving gases, the constant allows engineers and scientists to relate properties such as pressure, volume, temperature, and energy. This specific unit of kJ/kg K is significant because it accounts for the amount of energy required to raise the temperature of one kilogram of the gas by one Kelvin, which directly relates to the heat capacity of the gas.

In context, the other choices present different units that do not correctly describe the characteristic constant for a gas or are incomplete. For instance, choices like J/g C or KJ/Kg/C pertain to specific heat capacities but do not appropriately match the specified parameters needed for the gas constant in this case. Hence, utilizing the kJ/kg K provides a precise and relevant measure for understanding the thermodynamic behavior of gases.